1. Field of the Invention
This invention relates to a method and device for controlling morphology of semiconducting polymers, methods for fabricating flexible devices, and flexible devices such as flexible Organic Field Effect Transistors (OFETs).
2. Description of the Related Art
(Note: This application references a number of different publications as indicated throughout the specification by one or more reference numbers in superscripts. A list of these different publications ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications is incorporated by reference herein.)
Flexible organic field-effect transistors (OFETs) based on solution-processed semiconducting polymers and polymer dielectrics are of considerable interest for state-of-the-art flexible “Plastic Electronics”1-6. However, charge carrier mobilities have remained below industrial requirements due to the difficulty of aligning semiconducting polymers on meta-stable (swellable) polymer dielectrics.1,7-9. As a result of the quasi-one-dimensional transport pathways of charge carriers along the backbone, charge transport in polymer semiconductors is limited by their nanomorphology10. Structural disorder, arising from the high degree of conformational freedom of polymer chains (causing chain folding, torsion, and structural defects) leads to electronic localization11. Thus, highly aligned polymer packing with minimized structural disorder is needed for achieving high mobility in conjugated polymers. Our recent progress toward this goal was reported using nanogrooved substrates to obtain chain alignment and associated anisotropy with resulting mobilities of 50 cm2 V−1 s−1 (and even higher) for regioregular polymers12-14, including poly[4-(4,4-dihexadecyl-4H-cyclopenta[1,2-b:5,4-b′]dithiophen-2-yl)-alt-[1,2,5]thiadiazolo-[3,4-c]pyridine] (PCDTPT; see FIG. 1a for the molecular structure). By employing a “sandwich” casting system comprising oleophilic nanogrooved dielectrics and glass spacers, semiconducting polymers were oriented parallel to the nanogrooves by capillary action14. The aligned polymer thin films exhibited strong anisotropy, showing more than 10-fold higher mobility for transport along the direction of alignment than perpendicular to the alignment. This concept of directed self-assembly of semiconducting polymers using nanogrooved substrates is also promising for achieving high mobility in solution-processed flexible OFETs.
Despite such high mobility, however, one finds that it is challenging to develop high mobility flexible OFETs using the capillarity of polymer solutions onto nanogrooved substrates because the nanogrooved SiO2 dielectric, which is a key component for inducing chain alignment, is a brittle material15. Therefore, a strategy for achieving high polymer alignment and high mobilities using a nanogrooved polymer dielectric, which is chemically and mechanically stable, is needed for the development of high mobility flexible OFETs. One or more embodiments of the present invention satisfy this need.